Fused bifocal contact lens unit

ABSTRACT

A fused bifocal contact lens having &#39;&#39;&#39;&#39;no jump&#39;&#39;&#39;&#39; characteristics at all points of the segment line separating the distant vision segment of zone from the near vision zone. The lens include conventional convex and concave surfaces on the exterior portions, but, by reason of the construction of the bifocal segment, the wearer does not experience double vision when his line of sight crosses the segment line and the wearer does not notice the presence of the bifocal segment when not looking therethrough. In one form, a full, concentric bifocal segment is provided, and in another form, a lens is provided in which the bifocal segment is of a bi-curve form, lies principally in the lower portion of the lens and is defined by an upper, outer curve and an inner curve forming a partially circular segment line across which there is no jump. Methods of forming such lenses, including the method of forming the near vision segment, forming a composite blank including both distant and near vision segments, and forming a bifocal lens from the composite blank, are described.

United States Patent Tsuetaki 154] F USED BIFOCAL CONTACT LENS UNIT [72]Inventor: George F. Tsuetaki, 445 Wellington, Chicago, 111 60657 22Filed: Jan. 19, 1970 21 Appl. No.: 3,683

[52] US. Cl. ..351/161, 351/177, 264/1 [51] Int. Cl ..G02c 7/04, G02c7/06 [58] Field of Search ..351/161, 177

[56] References Cited UNITED STATES PATENTS 3,270,099 8/1966 Camp..351/161 X 3,440,306 4/1969 Neefe ..351/161 X 3,472,581 10/1969Bronstein ..351/161 FOREIGN PATENTS OR APPLICATIONS 910,455 11/1962Great Britain ..351/161 OTHER PUBLICATIONS Mandell, Article inOptometric Weekly June 1, 1967 pgs. 19- 21 Dean, Article in Journal ofthe Texas Optometric Association Feb. 1969 2 pg.

[451 Aug. 15, 1972 Ruben, Article in British Journal of OpththalmologyVol. 50 1966 pgs 642 645 Primary Examiner-David H. Rubin Attorney-JamesT. Fitzgibbon [5 7] ABSTRACT I A fused bifocal contact lens having nojump characteristics at all, points of the segment line separating thedistant vision segment of zone from the near vision zone. The lensinclude conventional convex and concave surfaces on the exteriorportions, but, by reason of the construction of the bifocal segment, thewearer does not experience double vision when his line of sight crossesthe segment line and the wearer does not notice the presence of thebifocal segment when not looking therethrough. In one form, a full,concentric bifocal segment is provided, and in another form, a

lens is provided in which the bifocal segment is of a bicurve form, liesprincipally in the lower portion of the lens and is defined by an upper,outer curve and an inner curve forming a partially circular segment lineacross which there is no jump. Methods of forming such lenses, includingthe method of forming the near vision segment, forming a composite blankincluding both distant and near vision segments, and forming a bifocallens from the composite blank, are described.

7 Claims, 9 Drawing Figures PATENTEUAUB 15 m2 sum 1 or 2 INVENTOR GEORGEE TSUETAK/ Wfi 4 ,g;

JAE-M +19 Arry PATENTEU we 15 1972 SHEET 2 [IF 2 INVENTOR s mw E T wm TF. a m E G BACKGROUND OF THE INVENTION The present invention relates tobifocal contact lenses and particularly those of a fused construction,that is, lenses of the type in which the bifocal segment is formed as aninsert in the lens by being cast in place and joined to the remainder ofthe lens without the aid of the adhesives. Other types of bifocalcontact lens are known, particularly those in which, for example, thefront surface has a secondary or compound curve, that is, a differentcurvature at one part of the front surface than at another part. Lensesof this type do not form a part of the present invention, it being wellrecognized in the art that fused bifocal lenses are different in manyways from other, non-fused types of lenses.

With the advent of small diameter corneal contact lenses, greatlyincreased use of contact lenses has occurred. It is estimated that atleast 5,000,000 and perhaps more pairs of contact lenses are now in usein the United States alone. Up until the present time, the majority ofcontact lenses have been single vision types, although bifocal contactlenses in various forms are known in themselves. One problem hasexisted, however, which has been a drawbackto heretofore known types ofbifocal contact lenses, and this problem is referred to in the art asjump. The expression jump refers to the phenomenon which takes placewhen the line of sight crosses the segment line, that is, the boundarybetween the far vision and the near vision segments of the lens, and inparticular, the part of the boundary defining the inner edge of thebifocal segment. As a result of such jump, the wearer of bifocal contactlenses has had a somewhat annoying or unpleasant sensation when shiftinghis line of sight from one segment to the other.

As a result of this known drawback, there have been a number of effortsto solve this problem. So far, the efforts expended in this field havebeen known to succeed in providing a lens in which jump was eliminatedat only one point on the segment line, commonly the point thereof lyingat or just beneath the center of vision line of the eye, assuming thelens to be centered with respect to the eye and in its desired positionof use. However, in these constructions, the phenomenon of jump stilloccurs whenever the line of sight crosses the segment line at any pointother than the single point in question. Accordingly, in any task inwhich eye movement is predominantly from left to right, such as a typingjob wherein a typist views material to be typed in a series ofright-to-left eye movements, the phenomenon of jump is very annoying andtiring to the eyes. In addition, in a concentric bifocal lens, so-calledbecause the bifocal segment is a continuous ring of toroid extendingaround the far or distant vision segment, a scattering or halo effectresulting from segment line jump is commonly observed upon viewinglights at night, such as when driving an automobile. No systems havebeen known to have been produced, or even proposed on a theoreticalbasis, in which the jump could be eliminated entirely around the segmentline. Accordingly, there is a significant need for such a lens.

Another disadvantage of other known types of bifocal contact lenses isthat the material comprising the far vision segment has in some casesbeen formed from a higher index of refraction material, which isnotoriously unstable dimensionally. Lenses of this type have .also beendifficult to produce and are therefore undesirable from a manufacturingstandpoint.

For the foregoing reasons, and for other reasons, and because of theshortcomings of certain prior art cons'tructions in fulfilling thedesired characteristics of a said bifocal contact lens, it is an objectof the present invention to provide an improved bifocal contact lenscharacterized by a lack of jump throughout the entire extent of thesegment line or surface defining the inner portion of the bifocalsegment of the contact lens.

Another object is to provide an improved method for forming such lenses.

A still further objectis the provision of a contact lens of the fully nojump type in which the bifocal segment totally surrounds the central,distant vision segment.

' Another object is the provision of a bifocal contact lens having abifocal segment extending only partially around the far vision segment,and in which there is no jump across the entire extent of the innerportion of the segment line or surface.

Another object is the provision of an improved bifocal contact lenswhich is of particular advantage to users whose occupations call forfrequent lateral eye movement into and out of the near vision or bifocalsegment of the lens.

A further object is the provision of a lens which is dimensionallystable by reasons of its construction.

A still further object is the provision of a method for making suchlenses in which full advantage may be taken of existing materials toprovide lenses of characteristics not heretofore able to have beenachieved as a practical matter.

Another object is the provision of a bifocal contact lens in which thecenter of curvature of the front face of the bifocal segment lies on aline extending from the center of curvature of the lens rear or basecurve to the segment surface or line separating the distant visionportion centrally located in the lens from the near vision segment ofthe lens which is disposed radially outwardly of the distant visionsegment.

A still further object is the provision of a method of forming a contactlens wherein a cylindrical blank is rotated about a given center lineand a front curve imparted thereto by moving a tool in an are about apoint radially offset from said center line, in which an opening is thencut in the center of the resulting blank of a size desired to provide adistant vision segment, in which a mass of fluid material for formingthe remainder of the lens is cast in place covering at least a portionof said blank, including the front surface portion and the openingtherein to form a composite blank, and in which the resulting compositeblank is cut to form a contact lens such that rear base curve thereofcomprises portions of both near and far vision segments in which thefront surface comprises entirely material from which the far visionportion is made.

A further object is the provision of a method for forming a no jumpbifocal contact lens blank which includes forming a near vision blankaccording to a predetermined pattern, and covering a portion of the nearvision blank with material comprising a distant vision forming element,so that a lens having predetermined characteristics not heretoforeattainable may be cut from the resulting composite blank.

A still further object is the provision of a no jump bifocal contactlens having single radius front and rear curves.

Another object is the provision of a fused bifocal contact lens havingthe major or far vision portion thereof made from a lower index ofrefraction material than the material comprising the bifocal segment.

A still further object is the provision of a bifocal contact lens whichmay, by reason of the manner in which curves defining the bifocalsegment are formed, include a bifocal segment portion of greatly reducedthickness, thereby resulting in a lens having a greater proportion oflow refractive index material and greater dimensional stability.

A still further object is the provision of a no jump bifocal contactlens having a bi-curve or similar surface defining a portion of thebifocal segment of the lens.

Another object is the provision of a method which is equally suitablefor making no jump concentric bifocal lenses and no jump bifocal lenseshaving bifocal segments in the form of only a portion of a ring, anddefined in part by surfaces having at least two characteristics,different forms.

A still further object isthe provision of an improved method of makingno jump bifocal contact lenses as well as composite blanks from whichsuch lenses may be made.

The present invention accomplishes these objects,-

and others inherent therein, by providing a bifocal contact lens inwhich the center of curvature of the front surface of the bifocalsegment lies on a line extending from the center of curvature of thebase curve of the lens to that portion of the segment surface separatingthe distant vision segment of the lens from the inner edge of the nearvision segment, and by providing a method wherein a near vision segmentblank is formed with a front curve and a central opening therein, inwhich a distant vision portion is formed thereover to create a compositeblank, from which a finished lens may be made by forming a front lenscurve and a rear lens curve such that the center of curvature of thebifocal segment front surface will be on a line extending from thecenter of curvature of the rear lens surface to the inner surface of thebifocal segment.

The manner in which these objects, and other objects and advantagesinherent in the invention are accomplished will become more clearlyapparent when reference is made to the following detailed description ofthe preferred embodiments of the invention, and to the accompanyingdrawings in which like reference numerals represent corresponding partsthroughout;

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevational view ofone form of contact lens embodying the invention;

FIG. 2 is a vertical sectional view, of the lens unit of FIG. 1, takenalong lines 2-2 thereof;

FIG. 3 is a front elevational view of another form of lens unit madeaccording to the invention;

FIG. 4 is a vertical sectional view of the lens of FIG. 3, taken alonglines 4-4 thereof;

FIG. 5 is a view of a near vision segment blank being formed accordingto the invention;

FIG. 6 shows the blank of FIG. 5 with the front curve formed thereon andwith a central opening formed therein;

FIG. 7 shows the formation of a composite blank and the manner offorming a portion of the base curve thereof;

FIG. 8 shows the formation of the finished rear or base curve of thelens using the previously formed near vision segment blank as a partthereof; and

FIG. 9 shows one method of forming the front curve of the finished lensfrom the composite blank.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Although theprinciples of the present invention are applicable to a variety ofoptical devices, the invention will be described with reference to abifocal contact lens constructed of a transparent plastic material andbeing adapted to be worn over the cornea of the eye of the wearer. Asused herein, and in the claims, it will be understood that expressionssuch as front, rear, top, bottom, or other like words implying directionor orientation, are intended to apply to the orientation that such unitswould have when in position of use over the eyes of a person, it beingfurther understood that the lenses are useful in composition, and that aperson wearing the lens might have his head and eyes in any one of anumber of positions from time to time.

Referring now to the drawings in greater detail, FIG. 1 and 2 show oneembodiment of a contact lens unit 20 having a front surface portion 22and a rear surface portion 24 defining therebetween a solid lens body 26having a centrally disposed, distant vision segment 28 and a near visionsegment 30 of generally annular form therein. The near vision segment 30is separated from the far vision zone 28 of the lens 20 along agenerally annular, inner surface portion 32 of the bifocal segment 30and is further defined in part by a front bifocal segment surface 34 anda rear bifocal segment surface 36 which forms a portion of thecontinuous, smooth socalled base curve making up the rear surface 24 ofthe lens unit. The front surface 34 and the rear surface 36 of thebifocal segment 30 meet along an outer bifocal segment line 38 lying onthe rear surface 24 of the lens 20. Although the outer segment line 38is spaced apart from the outer edge 40 of the lens 20 by a margin 42 ofthe same material comprising the principal lens body or element 26 andthe far vision zone 28, in use, the eye is never positioned relative tothe lens such that the center of vision thereof lies within the margin42, and accordingly, the optical characteristics of this margin are notimportant. Nevertheless, since this part of the lens can be made from alow index of refraction, stable plastic material, the overalldimensional stability of the lens is greatly enhanced.

Referring now in particular to FIG. 2, it will be noted that the innersurface portion 32 of the bifocal segment 30 meets the front bifocalsegment surface 34 along a segment line 46. Although, properlyconsidered, the inner surface of the armular or toroidal bifocal segment30 of this embodiment is defined by two segment lines 44, 46 and thesurface portion 32 extending therebetween, the expression segment lineis sometimes applied to the surface 32 and the lines 44, 46 definingsuch surface, it being understood that the width of the surface 32 is sosmall that it may appear to be only a line, and, for practical purposes,may be regarded as such.

Furthermore, the shape of the inner surface portion 32 may be trulycylindrical, or may be in the form of a frustum of a cone, as will beappreciated by the fact that, strictly speaking, a surface entendingfrom the center of curvature of the rear surface 24 of the lens anddefining an opening therein of any significant size would be conical inshape. However, in view of typical dimensions of lenses of the typeunder consideration, it may be said that this surface 32 is, forpractical purposes, cylindrical.

Furthermore, being of only very slight axial extent, as pointed outabove, and because when viewed in elevation, it appears to be circular,it may sometimes be referred to as circular rather than as having athree dimensional form of circular cross-section.

By reference to FIG. 2, it will also be noted that an imaginary centerline 48 extends through the center of the far vision zone 28, and thatthe center of curvature 50 of the rear lens surface 24 lies thereon. Inthe illustrated embodiment, the center of curvature 52 of the frontsurface 22 of the lens 20 also lies on this line. However, as will bebrought out further herein, and as is well known in the art, this centerof curvature 52 may lie axially either in front of or behind the centerof curvature 50, or coincidently therewith. Moreover, it is known to becommon for the center of curvature 52 to lie somewhat radially outwardlyfrom the center line 48, to render one portion of the lens heavier thananother to provide orientation thereof in use, particularly where thevisual condition to be corrected by the lens differs in relation todifferent meridians of the eye. On the other hand, such orientation maybe accomplished by disposing a dense metal weight within the lens, suchas at the interface between the bifocal segment 30 and the body 26 ofthe lens, as described and claimed in US. Pat. No. 3,431,327.

Referring now to a very important feature of the invention, it will benoted that the front surface 34 of the bifocal segment 30 has a centerof curvature 54 which lies on a straight line 56 extending between theradially inner surface portion 32 of the segment 30 and the center ofcurvature 50 of the rear surface 24 of the lens 20.

As a result of this feature, two definite advantages are provided in thepresent invention. By locating the center of curvature 54 on line 56, nojump occurs as the eye moves so that the line of sight crosses thesegment line", that is, the lines 44, 46 and the surface 32 definedthereby.

In addition, the bifocal segment may be made much thinner, since greaterpower may be provided with less overall thicknessof the segment 30, andtherefore the lens may be made more stable dimensionally. In otherwords, a greater relative difference in curvature between surface 24 and34 may be accomplished for the same thickness and radial extent ofsegment 30 than may be accomplished if the center of curvature 54 wereto lie on the center line 48, for example. Thus, a bifocal segmentshould provide the necessary power resulting from difference in thesecurvatures, but the segment must also be of a radial extent sufiicientso that the center of vision will fall therein during any amount ofmovement is it reasonably anticipated that the eye will make while thecenter thereof is in the near vision zone. Bifocal contact lenses formedas illustrated in FIGS. 1 and 2 have been found to be free from theobjectionable jump" phenomenon and can be made with the bifocal segmentsthereof having much thinner cross-sections than has heretofore beenpossible.

Referring now in particular to FIGS. 3 and 4, another form of theinvention is shown. FIG. 3 shows a lens 20a which is similar to lens 20of FIGS. 1 and 2, differing therefrom only in the construction of thebifocal segment 30a. As can be noted by reference to FIGS. 3 and 4, alens body 26a defined by front and rear curved surfaces 22a, 24a isshown, with a center portion 280 provided therein for distant viewing.As in the earlier described embodiment, the center of curvature 50a liesalong an imaginary line 48a passing through the center of the far visionsegment 28a, while the center of curvature 54a of the front surfaceportion 34a of the bifocal segment 30a lies along a straight line 56aextending from center of curvature 50a to the surface 32a defined bysegment lines 440, 46a.

An outer segment line 38a extends around the outer edge of the bifocalsegment 30a, spaced inwardly from the lens edge 40a by the margin 42a.By particular reference to FIG. 3, however, it will be noted thatsurface 32a is in the form of only a portion of complete ring orannulus, and since the outer segment line 38a is also in the form ofonly a portion of a circle, the bifocal segment 30a itself is in theform of a part of a ring of toroid. A pair of generally radiallyextending, somewhat arcuate surfaces 58, 60 extend between the ends 62,64 of the surface 32a outwardly toward and terminate in the comers oredges 66, 68 of segment line 38a. Accordingly, the bifocal segment isdefined by inner surface 32a, by a pair of surfaces 58, 60 ofcurvilinear form but larger radius than the other surface 32a definingthe radially inner parts thereof, as well as being defined at its outeredge by the segment line 38a formed where segment 30a joins the rearsurface 24a of the lens 20a.

By reason of the placement of the center of curvature 54a in relation tothe straight line 56a extending between the center of curvature 50a ofthe rear surface 24a and the inner segment surface 320, no jumpcharacteristics are provided at all points of the segment lines andsurfaces 44a, 46a, and 32a. Since there is no bifocal segment abovesurfaces 58, 60, jump is obviously likewise not experienced if the eyemoves upwardly from the center of the lens and radially outwardly.

A lens of the type shown in FIGS. 3 and 4 is of great utility for thosewho wish to have a large area distant vision segment 70 in addition tothe distant vision segment provided by the partially circular region28a, but who also wish to have a substantial bifocal segment acrosswhich the center of vision of an eye may pass, particularly laterally,without experiencing jump. In reference to FIG. 3, it will be noted thatone preferred form of the invention shows that surfaces 58, 60 arecurvilinear; it will be understood, however, that these surfaces mightbe of a somewhat different form.

Lenses made in the form shown in FIG. 3 are particularly advantageous toworkers who wish to have a large single vision or far vision segment,but whose eye movements are customarily largely lateral within the nearvision sight zone or area. Thus, for example, typists commonly read frommaterial positioned to the right or left of the typewriter and scan thematerial from left to right with occasional or frequent glances to thestraight ahead position of the typewriter, all while looking somewhatdownwardly.

Since all of this viewing is desired to be done through the near visionsegment, it is necessary that the bifocal segment have considerablelateral extent, and at the same time, entry from the distant vision zoneto the near vision zone should always desirably be across a surface orline wherein there is no jump. Lenses such as those shown in FIG. 3 arethe only known type which will provide a combination of all theseadvantageous features, particularly in a dimensionally stable lens whichis easy to manufacture.

In the past, it was not believed possible to provide a fused bifocalcontact lens in which there was no jump at any part of the segment lineor surface separating the near vision zone from the far vision zone, andonly a few lenses have been provided in which there was no jump at onlya single point on the segment line. In addition, in lenses wherein theprincipal body of the lens was manufactured from a high index ofrefraction material and the bifocal segment was made from a lower indexmaterial, the resulting lens was inherently unstable dimensionally byreason of this fact. Furthermore, certain prior art lenses weredifficult to make by techniques similar or analogous to those normallyused in making fused contact lenses.

Accordingly, as pointed out above, an object of the present invention isto provide a method of making a lens which is practical from aproduction standpoint, and a method of making a blank from which suchlenses can subsequently be readily made. This method is illustrated inFIGS. 5-9. Thus, referring now to-FIG. 5, there is illustrated a bifocalsegment blank 100 comprising a body 102 of a transparent, high index ofrefraction plastic material, such as an acrylic plastic or the like,held on a rotatable mount 104 able to be turned by a shaft 106. A frontcurve 108 is imparted to the body 102 by swinging a cutting tool 1through an are as shown by the arrows in FIG. 5, about a point 112, bymounting the tool 110 on a lever arm 114. Thus, having in mind thedesired radius of curvature to be formed on the front surface 108 of theblank 100, and knowing the desired radius of curvature of the rearsurface of the lens ultimately to be formed from the blank, and alsoknowing the desired diameter of the central portion of far vision zone,it will be possible to determine the point 118 which will form thecenter of curvature of the rear surface of the lens, and in additionthereto, the radial offset of point 112 will be predetermined, sincethis point will fall on a straight line from point 118 to a pointdesignated 120 which represents the ultimate diameter of the opening inthe blank 100. Thus, point 112 is spaced inwardly from point 120 adistance equal to the radius of curvature of surface 108, and is offsetfrom the imaginary center line 1 16 an amount determined by the angle ofthe line 122 extending from the proposed center of curvature 118 of therear surface to be formed and point 120 on surface 108. Obviously, point120 will lie on the surface defining the radially inner edge of thebifocal segment.

After such front surface 108 has been formed and polished according tothe criteria set forth above, such cutting and polishing per se beingcarried out in a known manner, a tool 124 having cutting edges 126, 128is used to form an opening 130 defined by a continuous annular side wall132, terminating at point 120. Thereafter, as shown in FIG. 7, a mass132 of a transparent optical material having a lower index of refractionis cast or otherwise formed over surface 108 and filling opening 130, toform a composite blank 134. This material is also typically an acrylicplastic, differing from the plastic forming body 102 only in chemical orphysical characteristics, particularly its index of refraction. Thisblank 134 is then held in a collet 136 for rotation about its centralaxis, since a rough base curve 136, that is, the curve of the radiuswhich will form the rear surface of the finished lens is formed usingpredetermined point 118 as the center thereof. In FIG. 7, tool 138 isillustrated as performing this function.

Referring now to FIG. 8 it will be seen that a final cut is made by acutting or polishing tool somewhat diagrammatically shown at 140, so asto establish a final curvature 142 on the rear surface of the lens.Feeding of the composite blank axially of the center line 116 is stoppedwhen the outer segment line 38 (or 380), lies outwardly of point 120 bya distance equal to the desired radial extent or width of the bifocalsegment 30 being thus formed. In this connection, it will be noted thatthe width or diameter of blank 100 is significantly greater than theintended final diameter of the entire bifocal segment 34. Thus, a blankhaving the rear surface finished in this manner will have the frontcurve of the bifocal segment established and the rear or base curve ofthe entire lens formed according to the prescription desired. Thebifocal segment will then be of the proper width, and the openingdefined for distant vision will have been determined by the diameter ofthe opening 130, therefore, the only remaining step is a step such asthat shown in FIG. 9, wherein a cutting tool is used to cut the ultimatecontour of the front surface 144 of the finished lens. If it is desiredto make the lens of the shape shown in FIG. 9, for purposes of havingone portion thereof heavier than a diametrically opposite portion, thecenter of curvature 146 of the front surface 144 is offset from thecenter line 116 as shown.

Manufacturing a bifocal lens of the type shown in FIG. 3 is carried outin the same manner as just described, except that, after completion ofthe step shown in FIG. 6, the body 102 of the bifocal blank 100 is cutso that, when viewed from the front, it will have the profile of thebifocal segment 30a shown in FIG. 3, for example. Thus, with the segmentbeing in the form of a partially cylindrical section with a partiallycircular central opening therein, the far vision portion is cast andmanufacture of the lens is thereafter carried out as shown in FIGS. 7, 8and 9. Thus, the method illustrated in FIGS. 5-9 is applicable to lensesin which the bifocal segment is ultimately of the form of a completering or toroid, or of the form of only a portion of a ring or toroid.

In reference to the method just described, it will be noted that oneadvantage of the method is the fact that the curves cut in the lens aresurfaces of revolution having simple forms. It is not necessary, inorder to obtain the no-jump feature and other advantages of theinvention, to provide compound curves which are difficult to form andwhich, if lying on a front surface of the lens, would be furtherdisadvantageous.

It will thus be seen that the present invention provides a new bifocalcontact lens, lens blank, and method having a number of advantages andcharacteristics including those pointed out herein and others which areinherent in the invention.

1 claim:

1. A bifocal contact lens unit having a generally convex front lenssurface portion and a generally concave rear lens surface portion andincluding a distant vision portion and a bifocal segment portion fusedin place within said lens unit and partially defined by a segment frontsurface portion having a given radius of curvature and a plurality ofcenters of radial curvature, and an at least partially annular,generally axially extending, generally radially inwardly facing innersegment surface portion meeting said rear lens surface along a givenline and meeting said segment front surface along a segment line, eachof said centers of radial curvature lying on a given sight line betweena center of curvature of said rear lens surface portion and the point onsaid segment line intersected by said given sight line, whereby saidlens is characterized by the absence of image displacement when thesight line of a user crosses any portion of said radially inner surfaceof said bifocal segment at any point along said inner segment surfaceportion.

2. A lens unit as defined in claim 1 in which said at least partiallyannular inner segment surface portion is completely annular, saidbifocal segment being fully annular and totally surrounding said distantvision portion of said lens.

3. A lens unit as defined in claim 1 in which said partially annularinner segment surface portion extends through an arc of about one-halfof a circle, said bifocal segment having edge surfaces extendingradially outwardly from the ends of said inner surface portion andoccupying about one-half of said complete lens unit when viewed from thefront.

4. A lens unit as defined in claim 1 in which said given radius ofcurvature of said segment front surfac portion is a constant radius ofcurvature.

5. A lens as defined in claim 1 wherein said center of curvature of saidrear lens surface portion is a single center of curvature.

6. A lens as defined in claim 1 in which said lens front surface portionhas a center of curvature lying at a point offset from a sight lineextending from said rear lens surface center of curvature to the centerof said lens unit.

7. A lens as defined in claim 1 in which said bifocal segment is furtherdefined by a radially outer edge portion lying generally parallel to theouter edge of said lens, wherein said segment line occupies aboutone-half of a circle, wherein said radially outer edgeportion of saidbifocal segment subtends a greater angle than said segment line, andwherein generally radially disposed segment lines extend betweenrespective outer ends of said segment line and said outer edge portions.

1. A bifocal contact lens unit having a generally convex front lenssurface portion and a generally concave rear lens surface portion andincluding a distant vision portion and a bifocal segment portion fusedin place within said lens unit and partially defined by a segment frontsurface portion having a given radius of curvature and a plurality ofcenters of radial curvature, and an at least partially annular,generally axially extending, generally radially inwardly facing innersegment surface portion meeting said rear lens surface along a givenline and meeting said segment front surface along a segment line, eachof said centers of radial curvature lying on a given sight line betweena center of curvature of said rear lens surface portion and the point onsaid segment line intersected by said given sight line, whereby saidlens is characterized by the absence of image displacement when thesight line of a user crosses any portion of said radially inner surfaceof said bifocal segment at any point along said inner segment surfaceportion.
 2. A lens unit as defined in claim 1 in which said at leastpartially annular inner segment surface portion is completely annular,said bifocal segment being fully annular and totally surrounding saiddistant vision portion of said lens.
 3. A lens unit as defined in claim1 in which said partially annular inner segment surface portion extendsthrough an arc of about one-half of a circle, said bifocal segmenthaving edge surfaces extending radially outwardly from the ends of saidinner surface portion and occupying about one-half of said complete lensunit when viewed from the front.
 4. A lens unit as defined in claim 1 inwhich said given radius of curvature of said segment front surfaceportion is a constant radius of curvature.
 5. A lens as defined in claim1 wherein said center of curvature of said rear lens surface portion isa single center of curvature.
 6. A lens as defined in claim 1 in whichsaid lens front surface portion has a center of curvature lying at apoint offset from a sight line extending from said rear lens surfacecenter of curvature to the center of said lens unit.
 7. A lens asdefined in claim 1 in which said bifocal segment is further defined by aradially outer edge portion lying generally parallel to the outer edgeof said lens, wherein said segment line occupies about one-half of acircle, wherein said radially outer edge portion of said bifocal segmentsubtends a greater angle than said segment line, and wherein generallyradially disposed segment lines extend between respective outer ends ofsaid segment line and said outer edge portions.